1. Field of the Invention
The present invention relates to a circuit for issuing a loop detection signal in a subscriber circuit having an on-hook transmission facility.
Known in the art is a subscriber circuit of an exchange which employs "on-hook transmission", that is, issues information such as the telephone number of the calling subscriber in the form of a multifrequency signal or other AC signal during the idle intervals of the ringer. In a subscriber circuit in which such an "on-hook transmission" method is used, it becomes necessary to perform loop detection without error when the called subscriber responds to the ringer and picks the phone up off the hook.
2. Description of the Related Art
As will be explained in detail later by referring to the drawings, there is a possibility of an erroneous loop detection signal being produced. Such erroneous detection must be prevented. For this purpose, conventionally the following procedure was adopted: Namely, when the on-hook transmission signal became "1", a bias voltage was applied to the A-line and the B-line and the threshold value Vth of the loop detection was changed in the same way as the change of the potential of the A-line and the B-line due to the time constant of the battery feed circuit and subscriber line at this time. By this, erroneous detection was prevented.
However, according to such a conventional method, it is necessary to add a time-constant circuit comprising resistors and capacitors to the circuit for generating the threshold value Vth of the loop detection circuit and further to make that time-constant match the time-constant when the bias voltage is applied to the A-line and the B-line. As a result, there arose the problems that the number of parts was increased and the cost was increased due to the need for improvement of the precision of the parts.